The present invention relates generally to telephone transmission and, more particularly, to the transmission of high-speed digital signals between a telephone central office and the customer premises.
Perhaps the most flexible and least expensive approach for transmitting data over telephone lines is to use the existing voiceband telephone channels normally used to carry speech. The channel between the transmission endpoints may be either a switched network connection--established by the user at one endpoint by simply dialing the telephone number of the other endpoint--or it may be a permanent, private line connection which is set up for the user by the telephone company. In either case, once the connection has been established, data from the user's data communication/processing equipment is input to a voiceband modem which generates an output analog line signal having a frequency spectrum which matches the passband of the voiceband telephone channel. At the receiving end, a matching modem recovers the data from the received line signal and passes it to user's equipment at that end.
For a given level of noise and distortion, the rate at which data can be communicated over a channel is limited by its bandwidth. The bandwidth of the typical voiceband telephone channel is about 4 kHz. For typical levels of noise and distortion, this limits the transmission rate over such channels to a theoretical maximum of about 23 kb/s (kilobits per second). For many applications--such as database input/retrieval or other applications typically involving a human being at at least one end of the transaction--data rates well below this theoretical maximum are wholly satisfactory. Indeed, a vast number of modems operating at bit rates ranging from 1.2 to 19.2 kb/s are in current use in a wide range of applications.
For many other applications, however, such as computer-to-computer file transfers, videotext, transmission of digitized speech or video, etc., voiceband telephone data transmission is unacceptably slow. Advantageously, most of the transmission facilities interconnecting telephone switching offices around the country communicate their information in the form of multiplexed, high-speed digital bit streams. These facilities can be configured to provide not only the standard 4 kHz voiceband channels, but also wideband channels capable of carrying customer data at, for example, the so-called DS-1 rate of 1.544 Mb/s (megabits per second) or even higher.
The challenge, however, is to get the customer's high-speed data to the central office from the customer premises. In the future, it is anticipated that this need will be met by optical fiber ubiquitously linking the customer premises with the central office. It will be well into the twenty-first century, however, before this promise becomes a reality. At present, then, and for the immediate future, the existing telephone local distribution system--comprised of copper wire pairs--is and will continue to be the mechanism for delivering high-speed data to the central office.
Indeed, telephone engineers have been successful in providing transmission schemes that allow for high-speed data transmission from the customer premises to the central office. In the mid-1970's, for example, AT&T introduced a digital data communications network--the Digital Data System (DDS)--in which data at rates of up to 56 kb/s was transmitted from the customer premises to the central office using a four-wire local circuit, i.e., two two-wire pairs. The essence of the transmission scheme was to use bipolar baseband transmission in combination with, inter alia, fixed equalization to compensate for linear distortion and thereby provide a channel with flat loss up to frequencies sufficient to transmit at the required bit rate. This scheme allowed for unrepeatered transmission of almost eight miles at the 56 kb/s rate (and even greater distances at lower rates), thereby truly providing high-speed customer-premises-to-central-office transmission over the "local loop" for a significant base of customers. (See, for example, E. C. Bender et al, "Digital Data System: Local Distribution System," The Bell System Technical Journal, Vol. 54, No. 5, May-Jun. 1975.)
Subsequently, a 1.544 Mb/s speed was added to DDS, and data transmission at that rate was thereafter provided in other digital data transmission offerings. This transmission rate was achieved by using the technology developed for the so-called T1 carrier system--which had to that point been principally used to interconnect telephone central offices. Here again, the transmission scheme involved a four-wire circuit and a bipolar transmission format. Indeed, the design of DDS was based on the previously existing T1 technology. At the 1.544 Mb/s rate, however, compensation for channel distortion and noise required equalization and regeneration of the line signal at no more than every 6000 ft (6 kft).
The above approaches are certainly technically sound and are used quite extensively. They are in various ways, however, not up to the demands of the 1990's. For example, the keystone of telecommunications in the coming decade will be the Integrated Services Digital Network (ISDN)--a telecommunications facility that will provide, using (a) a unified addressing and signaling scheme and (b) a single physical point of access, the capabilities that are now provided by a host of separate networks, such as voice, circuit-data, packet-data, telex, private-line networks, etc. Central to the implementation of ISDN is the notion of completing the digitalization of the telephone network by providing the customer with duplex, i.e., simultaneous two-directional, digital transmission capability to the central office over a single two-wire pair at a distance of up to 18 kft at speeds ranging from the so-called ISDN "basic" (2B+D) rate (with framing, maintenance and control bits) of 160 kb/s up to the so-called "primary" (23B+D) rate (again with framing, maintenance and control bits) of 1.544 Mb/s and even beyond.
Disadvantageously, transmission based on T1 technology, although usable in this application, is relatively expensive to provision and maintain. This is principally due to the requirement for closely spaced regenerators and, secondarily, to the requirement of a separate two-wire line for each direction of transmission. Success of ISDN, by contrast, demanded a low-cost solution, at least for transmission at the 160 kb/s "basic" rate. A DDS type of transmission scheme, by contrast, could be used in ISDN applications, but is not feasible for data rates much in excess of the ISDN basic rate.
Ultimately, telephone engineers chose the solution described, for example, in document T1D1.3/86-145R1 dated Oct. 13, 1986 prepared by the CCITT Ad Hoc Group on Draft Standard and entitled "Draft Standard for ISDN Basic Access Interface for Application at the Network Side of NT1, Layer 1 Specification." Basically, this approach uses a four-level pulse amplitude modulation (PAM) transmission scheme, referred to as 2B1Q (because it maps two bits into one quaternary symbol) in combination with (a) adaptive equalization, which is a much more powerful technique for compensating for linear distortion in the channel than the fixed equalization previously used and (b) echo cancellation, which permits duplex transmission over a single wire pair. This approach, although providing two-wire data transmission over the local loop at the basic rate, pushes the state of the art for transmission of high-speed data from the customer premises to the central office. Where data rates significantly in excess of 160 kb/s over an 18 kft loop are needed, it has to this point been envisioned that T1 technology will have to be continued to be used.